There is a rather extensive literature suggesting that humans value egalitarian outcomes [1]}, [2]}. One piece of evidence comes from experiments on ultimatum-bargaining [3]}. Here, participants typically reject mutually beneficial offers if these offers put them at a notable disadvantage. Another piece of evidence comes from experiments on public good games with punishment [4]}. Here, a non-negligible fraction of participants is willing to pay a cost to reduce the payoff of free-riders, even if any long-run advantage of punishment is excluded by the experimental design. While there is considerable cross-cultural variation in the extent to which humans reject unfair offers and retaliate against free-riders [5]}, [6]}, an aversion against disadvantageous inequity seems to be universal [7]} and it is developed by children at a relatively young age [8]}. The brain regions that are activated when rejecting unfair offers include the ventral striatum and the ventromedial prefrontal cortex [9]}. The former is associated with social comparisons, whereas the latter integrates costs and benefits into a decision value [2]}.
